Selenium (Se) exists in four major oxidation states, +6, +4, 0, −2 (Se(VI), Se(IV), Se(0) and Se(−II)). Its environmental speciation can be complex and dependent on various factors including availability of oxygen, the presence of other oxidizing or reducing species and pH, with a variety of both inorganic and organic species occurring within its biogeochemical cycle. Selenium is found in industrial waters in all four oxidation states, with the main species being selenate Se(VI) and selenite Se(IV). Reducing the levels of dissolved selenium in industrial wastewaters is becoming an increasing environmental concern, with new environmental regulations requiring that selenium levels in industrial discharges be reduced to ppb concentrations.
A variety of systems are known for biological selenium removal from water, typically using bacteria capable of reducing selenates and selenites to insoluble elemental selenium (see for example International Patent Application WO 2007/146658). These processes may require heat to be added to facilitate the biological process. Alternative processes that have been applied to Se(VI) removal include membrane separation (see for example International Patent Application WO 2012/040525), and reverse osmosis or nanofiltration combined with evaporation and crystallization.
Other options for removal of selenium from water involve the use of iron. Iron (Fe) also exists in a number of oxidation states, −2 to +6, with +2 (ferrous (Fe(II)) and +3 (ferric Fe(III)) being the most common Like selenium speciation, iron speciation is affected by the presence/absence of oxygen and other oxidizing or reducing species, and pH, amongst other factors. Biotic or abiotic redox reactions in the environment can form green rusts, which are metastable precipitates of mixed ferrous and ferric hydroxides with interlayers of sulphate or other anions.
Selenite Se(IV) removal from wastewater has been demonstrated through precipitation with ferric solids (typically oxy-hydroxides). Existing selenate Se(VI) removal techniques however are dependent on reduction of Se(VI) to Se(IV) or elemental Se(0) prior to removal from solution and, as such, are associated with long retention times (in the order of hours), high cost and large volumes of waste by-product. Co-precipitation of selenate Se(VI) with green rust under anoxic laboratory conditions has been reported as an initial step in the first-order kinetic reduction of Se(VI) to Se(IV) by green rust, with the rate constant for the reduction step being in the range 1.03-1.68 (×10−2 hour−1) (Myneni et al., 1997, Science, 278:1106-1109).
Selenium-laden industrial waters typically also contain sulphate, chloride, bicarbonate and nitrate in concentrations that are often orders of magnitude greater than that of selenium, which can further complicate the removal of the selenium. Often, it may not be necessary to remove these additional constituents, so selective removal of selenium species in the context of relatively high concentrations of chemically similar anionic species may be advantageous in these cases.
U.S. Pat. No. 6,235,204, describes a process applicable primarily to Flue Gas Desulphurization (FGD) scrubber blow-down water, which is an improvement to the process described in an earlier patent (U.S. Pat. No. 4,806,264). Both processes rely on chemical reduction of selenium oxyanions using ferrous iron followed by precipitation of reduced selenium species (selenite/elemental selenium) with the generated iron sludge. The processes are subject to retardation of selenium chemical reduction (and thus removal) when inhibiting constituents such as oxygen gas, nitrate, bicarbonate, calcium and magnesium are present in solution. The process described in U.S. Pat. No. 6,235,204 also requires that reaction tanks are sparged with an inert gas (preferably nitrogen) in order to eliminate oxygen, which inhibits selenium reduction reactions. The minimum retention time required for selenium reduction/co-precipitation by the processes described in U.S. Pat. Nos. 6,235,204 and 4,806,264 is 20 and 120 minutes, respectively. Ferrous required for selenium reduction is provided from a ferrous salt (preferably ferrous chloride). A temperature in the range of 25-50° C. is required, with 35° C. being preferred.
U.S. Pat. No. 8,080,163 describes a process for treating wastewater to remove dissolved contaminants, which may include selenate, by a series of steps including primary and secondary treatments to remove solids, addition of metal salt reagents followed by a tertiary treatment that may include ion exchange, and recycling of metal salt residuals from the tertiary treatment to an earlier point in the process. The availability of the metal for generating metal salt residuals may be improved by chemically reacting the metal or solution by various means, including precipitation or co-precipitation.
U.S. patent application Publication No. 2012/0241381 describes a two stage process for reducing the concentration of selenium in wastewater that includes a first stage comprising bulk removal of selenite from the wastewater by precipitation with ferric chloride, followed by a second stage treatment that includes a hydride generation process or ion exchange.
U.S. Pat. No. 5,322,600 describes a process for removing dissolved selenium from wastewater by passing the wastewater sequentially through an anode chamber, a sub-micron filter, a cathode chamber and another sub-micron filter. Elevated temperatures of 90-120° F. are required.
Baek et al. (2013, Chemical Engineering Journal, 215-216:678-684) describe removal of selenate from solution in a batch electrochemical system using a reactive iron anode and copper plate cathode in a bicarbonate medium. The anode generates ferrous hydroxide, which reduces the selenate to selenite followed by precipitation with ferric iron. Attempts to remove selenate from the solution with ferrous hydroxide without the application of a current in this system were only minimally successful, and required long retention times (6 hours).
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.